The present invention relates generally to therapeutic radiology and, more particularly, to devices and methods for brachytherapy.
As is known, radiation therapy refers to the treatment of diseases with radiation. Of particular interest is the treatment of tumors, including malignant tumors such as cancer. In radiation therapy, it is desired to destroy the malignant tissue while minimizing the exposure of medical personnel to radiation and minimizing radiation damage to other tissue, such as nearby healthy tissue.
"Brachytherapy", with which the present invention is particularly concerned, is such treatment at relatively short distances, typically 0-3 cm, between the radioactive source and the relevant tissue. "Brachytherapy" is a comprehensive term including therapy effected by interstitial, intercavitary, and surface applicators. Interstitial and intracavitary techniques are particularly advantageous where superficial or accessible diseased tissue is involved. In contrast, another form of radiation therapy, "external beam therapy", involves treatment at relatively large distances, e.g. 70-100 cm between the radiation source and the skin. With "external beam therapy" it is difficult to minimize damage to underlying normal tissues.
There are two general types of brachytherapy, respectively involving permanent implants and temporary implants. By way of example, for permanent implants radioactive seeds containing radon and iodine-125 have been employed. For temporary implants, radium, cesium-137, and iridium-192 have been employed.
The foregoing provides a very brief summary of the context of the present invention. However, it is believed that the significant advance of the present invention will be better appreciated in light of an historical summary, as follows:
A wide variety of radioactive elements (radioisotopes) have been proposed for therapeutic use. Only a relatively small number have actually been accepted and employed on a large scale basis. This is due at least in part to a relatively large number of constraining considerations where medical treatment is involved. Important considerations are gamma ray energy, half-life, and availability.
An element employed almost immediately after its discovery in 1898, and one which is still in common use despite certain highly undesirable properties, is radium. By way of example, the following U.S. patents are cited for their disclosures of the use of radium in radiotherapy: Heublein U.S. Pat. No. 1,626,338; Clayton U.S. Pat. No. 2,959,166; and Rush U.S. Pat. No. 3,060,924.
A significant advantage in the use of radium for many purposes is its relatively long half-life, which is approximately 1600 years. The significance of a long half-life is that the quantity of radiation emitted by a particular sample remains essentially constant over a long period of time. Thus, a therapeutic source employing radium may be calibrated in terms of its dose rate, and will remain essentially constant for many years. Not only does this simplify dosage calculation, but long term cost is reduced because the source need not be periodically replaced.
However, a particularly undesirable property of radium is the requirement for careful attention to the protection of medical personnel, as well as healthy tissue of the patient. This is due to its complex and highly penetrating gamma ray emission, for example a component at 2440 keV. To minimize exposure to medical personnel, specialized and sometimes complicated "after loading" techniques have been developed whereby the radioisotope is guided, for example through a hollow tube, to the treatment region following the preliminary emplacement of the specialized appliances required.
In the past decade, cesium-137, despite a half-life of only 27 years, much shorter than that of radium, has gradually been displacing radium for the purpose of brachytherapy, especially intracavitary radiotherapy. Gamma radiation from cesium-137 is at a level of 660 keV compared to 2440 keV for the highest energy component of the many emitted by radium. This lower gamma energy has enabled radiation shielding to become more manageable, and is consistent with the recent introduction of the "as low as is reasonably achievable" (ALARA) philosophy for medical institutions. By way of example, the following U.S. patents are cited for their disclosures of the use of cesium-137 for radiotherapy: Simon U.S. Pat. No. 3,750,653; Chassagne et al U.S. Pat. No. 3,861,380; and Clayton U.S. Pat. No. 3,872,856. The Rush U.S. Pat. No. 3,060,924, referred to above for its disclosure of a radium source, also discloses the use of cesium-137.
Even more recently, the radioisotope iodine-125 has been employed for radiotherapy, particularly for permanent implants. A representative disclosure may be found in the Lawrence U.S. Pat. No. 3,351,049. Iodine-125, as well as other radioisotopes disclosed in the Lawrence U.S. Pat. No. 3,351,049, differ significantly from previously employed radioisotopes such as radium and cesium-137 in that the energy level of its gamma radiation is significantly lower. For example, iodine-125 emits gamma rays at a peak energy of 35 keV. Other radioisotopes disclosed in the Lawrence U.S. Pat. No. 3,351,049 are cesium-131 and palladium-103, which generate gamma radiation at 30 keV and 40 keV, respectively. Radioisotopes having similar properties are also disclosed in the Packer et al U.S. Pat. No. 3,438,365. Packer et al suggest the use of Xenon-133, which emits gamma rays at 81 keV, and Xenon-131, which generates gamma radiation at 164 keV.
Experience with such low energy gamma sources in radiotherapy has demonstrated that very low energy gamma rays, as low as 35 keV, can be highly effective for permanent implants. Significantly, such low gamma ray energy levels drastically simplify radiation shielding problems, reducing shielding problems to a level comparable to that of routine diagnostic radiology.
However, along with gamma ray energy, another physical property of such radioisotopes which must be considered is half-life. As briefly discussed above, a long half-life is desirable in many respects, especially when such factors as dosage calculation and long term cost are considered. The low energy gamma emitters mentioned above, as well as various others heretofore proposed and employed, share the common property of relatively short half-life. For example, the half-life of iodine-125 is 60 days, and the half-lives of paladium-103 and cesium-131 are only 17 and 10 days respectively. The half-life of xenon-133 is approximately 5 days and that of xenon-131, 12 days.
(There is no intention herein to suggest that such radioisotopes with relatively short half-lives are not therapeutically useful. In fact, the contrary is true. As the various references identified above point out, a short half-life is essential for permanent implants. Such radioisotopes may be more or less permanently implanted or embedded in a patient, for example, interstitially, and will deliver a calculable radiation dose over a predictable period of time, after which the radiation decays to a relatively insignificant level.)
For other than permanent implants, such low energy gamma emitting radioisotopes have a number of drawbacks related to convenience of use. In particular, with the radiation level rapidly decaying, shelf life becomes an important consideration. In order to achieve a desired dosage level, the time of radioactive sample preparation relative to the time of therapy must be carefully controlled, with attendant complexity. Further, such sources must be frequently replenished, requiring a continuing expenditure.
Because of these drawbacks relating to short half-life, the use of low energy radioisotopes such as iodine-125 and radon is currently limited to permanent implants. In short, the now-recognized advantages from the standpoints of shielding and radiation control possible with low energy gamma emmitters have been outweighed by the drawbacks attendant to short half-life.